The present invention relates to a micromachined structure with a deformable membrane. It also relates to a method for making such a micromachined structure. In particular, it relates to a micromachined structure for a sensor operating at a high temperature and notably for a pressure sensor.
Microelectronics techniques enable miniature pressure sensors to be made by means of collective manufacturing methods. They therefore provide sensors with reduced dimensions and of a low cost. They allow a sensor and its associated electronics to be made on the same support.
Micromachined pressure sensors consisting of a silicon membrane with a thickness of a few tens of xcexcm are known. The pressure difference between both faces of the membrane may be detected by measuring the stresses upon insertion with piezoresistive gauges obtained by ion diffusion or implantation. These piezoelectric gauges have a high sensitivity and a large mechanical stability due to the single crystal structure of the silicon used. Electrical insulation between each gauge and with the substrate on which they are produced, is achieved through reverse junctions. This has the disadvantage of limiting the operating temperature range of the sensors to a maximum of 125xc2x0 C. because of the high leak current of the reverse junction and of introducing a high noise level (thermal noise and piezoelectric junction noise) which reduces the dynamic range.
In order to obtain a micromachined structure for pressure sensors operating at a high temperature, several solutions have been investigated.
A first solution consists of a structure including a silicon carbide film (SiC) serving as a deformable membrane and comprising detection components, wherein this SiC film is deposited on a machined silicon support for releasing the membrane. Such a structure has the disadvantage of being limited in temperature by electrical leaks of the junction exhibited by the Sixe2x80x94SiC interface from 200xc2x0 C. upwards.
A second solution, directly derived from the first, consists of inserting an insulating layer (silicon oxide) between the SiC film and the silicon support. In this case, the micromachined structure includes a membrane consisting of three superimposed layers: the SiC surface film, the oxide intermediate layer and a silicone layer remaining after machining of the silicon support. The presence of this insulating layer provides a structure for which electrical leaks are limited. However, beyond 500xc2x0 C. the silicon portion of the membrane looses its elastic properties and is deformed.
A third solution consists of making the micromachined structure from a bulk SiC substrate. This solution is both electrically and mechanically suitable. However, the making of the membrane proves to be difficult, as etching of silicon carbide is very difficult. Moreover, this solution is expensive because of the high manufacturing costs of silicon carbide substrates.
U.S. Pat. No. 5,165,283 discloses pressure transducers operating at a high temperature and more particularly transducers using silicon carbide. Silicon carbide is grown on silicon. The obtained material then has a large density of defects which will promote junction leaks and limit its temperature life.
The invention provides a solution to the disadvantages exhibited by the structures of the prior art. It enables a micromachined structure to be made including a membrane which remains elastic even at a high temperature, and the support of which consists of a material which is easy to etch. The support material does not therefore need to have particular elastic properties at high temperatures.
Furthermore, according to the invention, the detection components are not directly made in the membrane layer, so that it is possible to have a membrane layer selected for its elastic functions without any constraint on its electrical properties. Further, the detection components are electrically insulated from the membrane layer, so that electrical leaks from the junction may be prevented.
Accordingly, the object of the invention is a micromachined structure able to operate at a high temperature, including a deformable membrane secured on a support allowing it to be deformed, wherein the membrane comprises at least a membrane layer in a material which retains its elasticity for said high operating temperature, said membrane layer supporting components made in a semiconducting material for detecting the deformation of the membrane, wherein the support is made in a material allowing the membrane to be released by a microelectronics technique, characterized in that the membrane has an electrically insulating interface with detection components, consisting of an electrically insulating layer, and further includes a surface layer in a single crystal semiconducting material in which said detection components are made, said surface layer being a layer transferred onto the electrically insulating layer.
According to a first embodiment, the support is locally, totally or partly etched, in order to release the membrane. According to a second embodiment, the support is obtained by deposition on a matrix, by local etching or by planarization of this coat and removing the matrix. The support may be made in bulk material or not. Notably it may consist of a substrate of the SIC type or a silicon substrate or even a mullite substrate.
According to a preferred embodiment, the membrane layer is in silicon carbide, the detection components are in single crystal silicon carbide and the support is in silicon. In this case, the electrically insulating layer may consist of a silicon oxide layer.
The object of the invention is also a method for making by means of micromachining at least a deformable membrane structure secured on a support and able to operate at a high temperature, characterized it consists of the following steps:
forming, on a face of an initial substrate in a material providing release by means of a microelectronics technique, a membrane layer in a material retaining its elasticity for said high temperature,
transferring, on the membrane layer formed on said substrate, a surface layer of a single crystal semiconducting material, with interposition of an electrically insulating layer,
making, on the free face of the surface layer, detection components such as piezoresistive gauges to be used as means for detecting the deformation of the membrane,
making electrical contacts on said free face for connecting the detection components to electrical connection means,
releasing the membrane from said structure
The release of the membrane of a structure may be made by removing material from the other face of the substrate. This may also be done by removing a matrix with a complementary surface to the micromachined structure.
In the case of a collective manufacturing method, this method also includes a final cutting step for obtaining separate structures.
Formation of the membrane layer may be accomplished by deposition.
According a first alternative, the transfer of said surface layer of semiconducting material is obtained from a wafer of the same material wherein this surface layer has been defined by a layer of microcavities generated by ion implantation, said wafer being stuck on the membrane layer formed on the initial substrate, then being cleaved at the microcavity layer in order to only retain the surface layer on the membrane layer. According to a second alternative, the transfer of said surface layer of a semiconducting material is obtained from a wafer comprising a layer of the same material secured on a support substrate, the surface layer being defined in said layer of the same material by a microcavity layer generated by ion implantation, said wafer being stuck on the membrane layer formed on the initial substrate, then cleaved at the micro cavity layer in order to only retain the surface layer on the membrane layer. Cleavage of the wafer may be obtained through coalescence of microcavities resulting from a heat treatment. The thickness of the transferred surface layer may be completed and monitored by epitaxy of the same material making up this surface layer.
According to a third embodiment, the transfer of said surface layer of semiconducting material is obtained from a wafer comprising said surface layer secured on a support substrate said wafer being stuck on the membrane layer formed on the initial substrate, the substrate supporting the wafer then being removed in order to only retain the surface layer on the membrane layer. This removal of the support substrate may be performed by a method selected from chemical dissolution and etching.
Advantageously, for the three alternative methods, the sticking of the wafer on the membrane layer is obtained by molecular adhesion.
The electrically insulating layer interposed between the surface layer and the membrane layer may consist of a first electrically insulating layer formed on the surface layer and of a second electrically insulating layer formed on the membrane layer, wherein said first and second electrically insulating layers are selected in order to contribute to the sticking by molecular adhesion.
If the detection components are piezoresistive gauges, the latter may be made by a method selected form ion implantation and ion diffusion. The electrical contacts may be made by depositing a metal layer on the free face of the surface layer and by etching this metal layer.
Preferably, said removal of material is obtained by a method selected from mechanical machining and chemical etching.